Wow, 900cc single and 2400cc twin (1200 per pot).
To put this into context, engines back then didn't rev much and compressions were low,
so to get any serious torque (for heavier vehicles) the cylinder capacities were somewhat enlarged.
In the car world, the 60hp early FIAT was 10 litres in a 4 cylinder car.
And early Rolls were quoted as 3:1 compression.
All started with a crank handle ?
Or pedalling gear if motorcycle related...
Engine efficiencies rapidly increased from there, particularly from motorcycle developments.
So engine sizes in larger vehicles generally decreased, while motorcyclists often opined for
more power from smaller lighter engines.
We wander slightly from Rex motorcycles though.